For a piezoelectric ink jet head that uses the electrostrictive effect of a piezoelectric element as the drive power source and is employed in an on-demand type ink jet printer or the like, one having such a constitution is employed as a plurality of pressure chambers to be filled with an ink are disposed on one side of a plate-shaped substrate in the direction of surface thereof, each of the pressure chambers being provided with a nozzle for discharging the ink that communicates therewith, and piezoelectric actuator stacked on the side of the substrate whereon the pressure chambers are disposed.
The piezoelectric actuator is generally constituted from an electrically conductive oscillator plate of such a size that covers the plurality of pressure chambers, a piezoelectric ceramic layer that are separately formed in correspondence to the pressure chambers, and a plurality of individual electrodes which are formed separately in correspondence to the pressure chambers, being stacked in this order. The electrically conductive oscillator plate, together with the individual electrodes, sandwiches the piezoelectric ceramic layer so as to serve also as a common electrode for applying an electric field to the piezoelectric ceramic layer.
In the piezoelectric ink jet head described above, when an electric field of the same sense as the polarizing direction of the piezoelectric ceramic layer is applied between at least one of the plurality of individual electrodes that correspond to the pressure chambers and the oscillator plate, the piezoelectric ceramic layer that is sandwiched by the individual electrode and the oscillator plate contracts in the direction of the surface. Since the piezoelectric ceramic layer is fastened onto the oscillator plate, the region of the piezoelectric actuator to which the electric field is applied deflects so as to protrude toward the pressure chamber in accordance to the contraction, while the deflection compresses the ink in the pressure chamber, so that an ink droplet is discharged through the nozzle for printing.
Recently, in order to provide for the ever-decreasing nozzle pitch that corresponds to the resolution of printing in the ink jet printers which has been becoming higher, a piezoelectric ink jet head has been commercialized that employs a piezoelectric ceramic layer of such a size that covers a plurality of pressure chambers, instead of a plurality of piezoelectric ceramic layer which are formed separately corresponding to the pressure chambers.
In this piezoelectric ink jet head, when an electric field of the same sense as the polarizing direction of the piezoelectric ceramic layer is applied between at least one of the plurality of individual electrodes that correspond to the pressure chambers and the oscillator plate, a particular region of the piezoelectric ceramic layer that is sandwiched by the individual electrode and the oscillator plate contracts in the direction of the surface like individual piezoelectric ceramic layer that are formed separately. In accordance to the contraction, the region of the piezoelectric actuator to which the electric field is applied deflects so as to protrude toward the pressure chamber, while the deflection compresses the ink in the pressure chamber, so that an ink droplet is discharged through the nozzle for printing.
A so-called bimorph piezoelectric ink jet head is also proposed that comprises two piezoelectric ceramic layers stacked one on another.
In the bimorph piezoelectric ink jet head, at the same time the first piezoelectric ceramic layer is caused to contract in the direction of the surface, a second piezoelectric ceramic layer is caused to expand in the direction of the surface by applying an electric field of opposite sense to the polarizing direction, thereby making it possible to achieve a satisfactory deflection of the piezoelectric actuator toward the pressure chamber, with the electric field of lower intensity than in the conventional piezoelectric ink jet head that has only one piezoelectric ceramic layer (called the unimorph type piezoelectric ink jet head in contrast to the bimorph type).
The piezoelectric ink jet head is described in detail in, for example, Japanese Unexamined Patent Publication Nos. H04-371845-A2 (1992), H08-118630-A2 (1996), H08-118663-A2 (1996), 2000-141647-A2 (2000) and 2001-77438-A2 (2001).
However, since the bimorph piezoelectric ink jet heads described in these publications all have two piezoelectric ceramic layers formed separately in correspondence to the pressure chambers, there has been such a problem that it cannot provide for the smaller nozzle pitch that corresponds to the higher resolution of printing in the ink jet printers.
In order to provide for the smaller nozzle pitch, the first and the second piezoelectric ceramic layers must have dimensions that cover the plurality of pressure chambers. But this leads to a problem of complicated structure of the piezoelectric actuator.
When two piezoelectric ceramic layers that have dimensions that cover the plurality of pressure chambers are used while maintaining the electrode structure in the piezoelectric ink jet head described in the patent publication mentioned above, it may be conceived to use a piezoelectric actuator AC having either the structure shown in FIG. 10 or the structure shown in FIG. 11.
Among these, the piezoelectric actuator AC shown in FIG. 10 is constituted from first individual electrodes 90a formed separately corresponding to pressure chambers 92, a first piezoelectric ceramic layer 97a having such a size that covers a plurality of pressure chambers, second individual electrodes 90b formed separately corresponding to the pressure chambers 92, a second piezoelectric ceramic layer 97b having such a size that covers a plurality of pressure chambers, a common electrode 98a having a size that covers a plurality of pressure chambers 92 and a protective layer 99 having such a size that covers a plurality of pressure chambers for protecting the common electrode 98a from the ink, that are stacked on the substrate 91 in this order from a position far from the substrate 91.
The piezoelectric actuator AC is manufactured by printing an electrically conductive paste to form the second individual electrodes 90b on one side of a green sheet of piezoelectric material that would make the second piezoelectric ceramic layer 97b, stacking a green sheet of piezoelectric material that would make the first piezoelectric ceramic layer 97a thereon, firing the stack so as to bake the three layers described above, printing an electrically conductive paste to form the first individual electrodes 90a on one side of the stack, and printing an electrically conductive paste to form the common electrode 98a on the other side of the stack.
However, the second individual electrodes 90b undergo changes in the position within the plane and in shape as the green sheet of piezoelectric material expands and then shrinks significantly during the firing process; moreover, the changes cannot be observed from the outside after firing.
Thus there is such a problem that it is difficult to align the individual electrodes 90a, 90b with the pressure chambers 92 when printing the electrically conductive paste to form the first individual electrodes 90a in alignment and when bonding the piezoelectric actuator AC onto the substrate 1.
In addition, in order to connect lead wires to the second individual electrodes 90b, it is necessary to form the lead-out wire by printing at the same time as the printing of the electrically conductive paste to form the second individual electrodes 90b, or to form via holes in the first piezoelectric ceramic layer 97a and provide the wiring.
In the case of the former, however, there is such a problem that a space for printing the lead-out wire is required which makes it impossible to provide for the smaller nozzle pitch.
In the case of the latter, there is such a problem that the additional process for forming the via holes or connecting the wire makes the manufacturing process complex and also makes the structure of the piezoelectric actuator AC more complex.
The piezoelectric actuator AC shown in FIG. 11 is constituted from the first individual electrodes 90a formed separately corresponding to pressure chambers 92, the first piezoelectric ceramic layer 97a having such a size that covers the plurality of pressure chambers, the common electrode 98b having such a size that covers the plurality of pressure chambers, the second piezoelectric ceramic layer 97b having such a size that covers the plurality of pressure chambers 92, second individual electrodes 90c formed separately corresponding to pressure chambers 92 and the protective layer 99 for protecting the second individual electrodes 90c from the ink, that are stacked on the substrate 91 in this order from a position far from the substrate 91.
The piezoelectric actuator AC can be manufactured similarly to that described previously.
The first and second individual electrodes 90a, 90c can be printed and formed after firing the green sheet of piezoelectric material, and therefore can be aligned easily with each either and with the pressure chamber 92.
In order to connect leads to the second individual electrodes 90c, it is necessary to form the lead-out wire by printing at the same time as the printing of the electrically conductive paste to form the second individual electrodes 90c, or to form via holes in the first and second piezoelectric ceramic layer 97a, 97b and provide the wiring.
In the case of the former, there is such a problem that a space for printing the lead-out wire is required which makes it impossible to provide for the smaller nozzle pitch, similarly to that described previously.
In the case of the latter, there is such a problem that the additional process for forming the via holes or connecting the wire makes the manufacturing process complex and also makes the structure of the piezoelectric actuator AC more complex.
There is also such a problem that the positions where the via holes are formed in the piezoelectric ceramic layers 97a, 97b undergo unpredictable displacement in the direction of plane in the firing process, thus making the alignment difficult.